1. Field of the Invention:
This invention relates to a process for the electrolytic removal of a polyimide resin.
2. Description of the Prior Art
Polyimide resins are often used as insulating materials in electric or electronic apparatus, since they have excellent heat resistance and are also comparable, or even superior, to other plastics in mechanical, electrical and chemical properties. Printed wiring boards (PWB), flexible printed circuits (FPC), tapes for tape automated bonding (TAB), etc. are each manufactured by photoetching a copper film formed on a polyimide-resin base.
A copper-polyimide substrate used for making a PWB, FPC, or tape for TAB has usually been prepared by a laminating process, i.e. bonding a copper foil to a polyimide resin with an adhesive. The substrate prepared by the laminating process has, however, been found defective, since impurities, such as chlorine or sulfate ions, are adsorbed to the adhesive layer between the copper film and the polyimide resin during the etching of the copper film or the removal of a photoresist, and are likely to cause a serious problem, such as an insulation failure, particularly when conducting interconnections forming a circuit on the substrate are very close to one another.
Proposals have, therefore, been made to prepare an improved substrate using electroless plating to form a metal layer on the surface of a polyimide resin directly without the aid of any adhesive, or the like. A brief description will be made of a process proposed for preparing a copper-polyimide substrate by forming an electrolessly plated copper layer on the surface of a polyimide resin. The surface of the polyimide resin is rendered hydrophilic by etching in a solution containing an alkali and a reducing agent, and is caused to adsorb, for example, palladium or silver as a catalyst. Then, the resin surface is electrolessly plated with a solution containing copper ions, such as copper sulfate, whereby a copper film is formed on the resin surface. An electroplating layer of copper may be formed thereon, if required.
The metal layer in the substrate prepared by electroless plating as hereinabove described has a peeling strength of 1500 to 2000 gf/cm at 25.degree. C. which is by far higher than the range of 800 to 1000 gf/cm as has usually been desired in a printed wiring board, or a TAB tape. Its peeling strength at 25.degree. C. is, however, reduced to as low as 100 gf/cm if the substrate is left for only 10 seconds in a soldering bath held at 290.degree. C.
If the substrate is used to make, for example, a TAB tape, the resulting tape is of very low reliability, since the metal layer cannot withstand a temperature of as high as 300.degree. C. to which it is rapidly heated when the leads of the tape are bonded to IC chips, but lowers its peeling strength rapidly and peels off easily.
We, the inventors of this invention, have studied the possible cause for the low thermal shock resistance of the substrate prepared by electroless plating. We have found that a modified layer of low heat resistance is formed on the surface of the polyimide resin by the etching which is carried out before electroless plating, and that the layer is composed of a substance of the amide series which is soluble in a solution of cyanogen or a cyanogen compound used for plating the metal layer patterned to form a circuit on the substrate, and as a result of its dissolution, causes the metal layer to peel off the resin surface.
We have, therefore, proposed a method of preparing an improved copper-polyimide substrate, as is disclosed in United States Patent Application Serial No. 448,635 filed on December 11, 1989. This method is characterized by the heat treatment of an electrolessly plated substrate at a temperature of 120.degree. C. to 420.degree. C. before it is subjected to any thermal shock, as by soldering, or coated with a solution of cyanogen or a cyanogen compound. This heat treatment is intended for converting the modified layer to a thermally or chemically stable substance having an imide bond by the dehydrocondensation of the amide and carboxyl groups which the modified layer contains.
The proposed method has been found to be capable of producing an improved substrate. When it has been dipped in a soldering bath having a temperature of 315.degree. C. for 10 seconds, the metal layer does not substantially lower its peeling strength, but maintains a peeling strength of 1300 gf/cm which is more than satisfactory for any substrate for a TAB tape. Moreover, no peeling of the metal layer results from its dipping in a solution of cyanogen or a cyanogen compound. The improved heat and chemical resistance of the substrate, as well as the excellent electrical and mechanical properties which it inherently possesses, has made it possible to manufacture reliable products, such as PWB and FPC. A new problem has, however, been found to arise when the substrate prepared by the proposed method is used to make a TAB tape, as will hereinafter be pointed out.
The manufacture of a TAB tape using a copper-polyimide substrate including an electrolessly plated copper layer is usually carried out by a process which comprises applying a plating resist to the surface of the substrate, exposure and development, plating the exposed copper layer electrolytically with copper, removing the resist, applying a polyimide-resin etching resist to the whole surface of the substrate, exposure and development, and dissolving the polyimide resin in a solution containing hydrazine hydrate. When the substrate which has been heat treated in accordance with the proposed method is used to make a TAB tape, however, it has often been the case that the polyimide resin cannot be dissolved or removed satisfactorily, but that a discontinuous organic resin film having a thickness of, say, 0.01 to 10 microns remains on the surface of the substrate. The remaining film makes it difficult to plate the back surfaces of the copper leads with gold or silver completely and no proper bonding can eventually be achieved between the copper leads and IC chips.
As a result of the recent sophistication of technology relating to TAB, there is known a tape of a polyimide resin provided on both sides thereof with metal layers which are connected to each other through via holes to thereby form a signal layer and a ground layer, respectively, so that no noise may be produced during the transmission of high-frequency signals. The via holes are, however, formed by dissolving the polyimide resin. No complete connection between the metal layers is possible, if the resin is not satisfactorily dissolved to form the via holes, but leaves an organic film on the surface of any such metal layer.
Although it has been expected that the substrate prepared by the proposed method is useful for making a TAB tape, as well as a PWB or FPC, because of its excellent electrical, mechanical and thermal properties, the absence of any reliable technique for the removal of the polyimide resin still prevents the expected use of the substrate for making any satisfactory TAB tape.